Gram-positive bacteria are those that are stained violet by Gram staining, and lack an outer membrane unlike Gram-negative bacteria. In phylogeny, Gram-positive bacteria belong to the phyla Firmicutes, Actinobacteria, and Tenericutes. Both Firmicutes and Actinobacteria are characterized by the high amount of peptidoglycan in their cell walls. The former, Firmicutes, are low G+C Gram-positive bacteria whereas the latter, Actinobaceria, are high G+C content Gram-positive bacteria. The phylum Tenericutes lacks a cell wall.
Most pathogens in humans are known as Gram-positive bacteria. Representative among them are Streptococcus and Staphylococcus, both cocci (sphere-shaped bacteria). Other Gram-positive pathogens are bacilli (rod-shaped bacteria) and can be subdivided based on their ability to form spores. The non-spore formers are Corynebacterium and Listeria, whereas Bacillus and Clostridium produce spores.
Recently, increasing attention has been drawn to the correlation between extracellular vesicles released from Gram-negative bacteria and diseases caused by Gram-negative bacteria [Kuehn, M. J., Kesty, N. C., Bacterial outer membrane vesicles and the host-pathogen interaction. Genes Dev. 2005, 19, 2645-2655]. Gram-negative bacteria-derived extracellular vesicles are known to bud off from the outer membrane. Because Gram-positive bacteria lack the outer membrane, with the plasma membrane enclosed by the cell wall, little has been known about the release of extracellular vesicles from Gram-positive bacteria as well as about the pathogenicity of Gram-positive bacteria-derived extracellular vesicles.